1. Field of the Invention
The present invention relates to the generation of electron beams for use in microwave generators and, more particularly, to the generation of gyrating electron beams in a controllable manner particularly suited for use in a wide range of gyro-amplifiers and gyro-oscillators.
2. Description of the Prior Art
Gyro-amplifiers and gyro-oscillators, which are commonly referred to as gyrotrons, require an electron beam that is different from that which is normally employed in linear microwave tubes, such as klystrons and traveling-wave tubes. In general, and as is known in the art, in gyrotrons microwave energy is extracted from the beam rotational energy through electron orbital phase bunching resulting from resonant interaction between the electron gyration and the transverse component of the electromagnetic waves contained in the interaction circuit, sometimes referred to as cyclotron resonant maser instability. To maximize this resonant interaction process and enhance the efficiency of the gyrotron, it is necessary for the bem forming system, also known as gyrotron gun or gyro-gun, to accomplish the following three factors: (a) form the gyrating electron beam with a large transverse-to-axial velocity ratio, xcex1=vxe2x8axa5/vz typically between one and two; (b) achieve and control the axial velocity, vz, so as to have a low velocity spread in order to provide phase bunching stability; and (c) place the electron""s beam guiding center, rg, at the peak of the wave transverse electric field. The attainment of these three factors for all gyrotron device applications has not been accomplished by a single gyro-gun because of certain limitations.
First, the desired factors of the transverse-to-axial velocity ratio, xcex1=vxe2x8axa5/vz, and the desired position of the electron""s beam guiding center, rg, primarily determine the electron orbital parameter which is different for various gyrotron applications. The selection of these desired factors in a gyro-gun to satisfy a gyrotron device requiring a particular orbit, such as, a small-orbit (non-axis encircling), may not be suitable when the same gyrotron gun is used for another application requiring a different type of orbit, such as, a large-orbit (axis-encircling).
Second, the desired parameter of controlling the axial velocity vz spread is primarily of importance to the interaction circuit located at the output of the gyrotron gun and which circuit extracts microwave energy from the kinetic energy of the gyrating electron. The axial velocity spread is determined, in part, by the cathode of the gyrotron gun and the operation of the cathode. One approach to control the axial velocity vz spread is to reduce the cathode""s annulus width which, in turn, has the disadvantage of creating higher cathode loading. Another approach is to increase the cathode""s mean radius which, in turn, has the disadvantage of increasing the overall size of the gyrotron gun which may not be desired for some applications.
In relation to the first problem, several approaches, dictated by the parametric requirements in the beam-wave interaction region, have been used to provide for a desired small-orbit or large-orbit gyrotron beam. Although gyrotron guns designed for a particular parametric requirement serve well their intended function, once built employing existing beam-forming techniques, the gyrotron gun is often difficult to adjust in order to accommodate parameter changes that may arise from time to time. Various beam-forming devices determined by parametric requirements that have been developed prior to 1981 are well documented and summarized in a detailed report by Baird and Attard entitled xe2x80x9cGyrotron Gun Study Reportxe2x80x9d of the Naval Research Laboratory (NRL) Report TR-3-476 (1981). Each of the approaches prior to 1981 is suitable for use as a beam-forming system for a specific gyrotron device, depending upon the type of beam parameters required. For instance, the gyrotron gun, magnetic injection gun (MIG), originally conceived in the early 1960""s, has been continuously used until the present time as a gyrotron beam-forming system and is particularly suited for small-orbit applications, but is not suited as a gyrotron beam-forming system having large-orbit applications. For a large-orbit gyrotron applications, a modified version of a magnetically shielded, space-charged limited Pierce gun (known in the art) has been proposed and is described by G. P. Scheitrum; R. S. Symons; and R. B. True, in the technical article entitled xe2x80x9cLow Velocity Spread Axis Encircling Electron Beam Forming System,xe2x80x9d documented in the Technical Digest of Electron Devices Meeting, pp 743-746 (1989). Accordingly, although various beam-forming techniques are known to accommodate both small and large-orbit gyrotron devices, no one technique is known to accommodate both the small and large-orbit applications.
In relation to the second problem, a primary cause of axial velocity vz spread in gyrotron devices is due to the fact that electrons emitted from the cathode of the gyrotron gun at different radial positions enclose different amounts of magnetic flux, commonly referred to as canonical angular momentum spread. As previously mentioned, several approaches are known to reduce the axial velocity vz spread and one of which is to reduce the cathode""s annulus width. This is not however very practical, since this reduction creates a higher cathode loading factor, which has a tendency to overburden the cathode and, thereby, degrade its operational life characteristic. Another approach is to increase the cathode""s mean radius. While this approach reduces velocity spread, it is accomplished at the expense of increasing the overall size of the gyrotron gun which may not be desired for some applications. An approach is to reduce the axial magnetic field on the surface of the cathode. An adaptation of this approach is to use a magnetic envelope and a magnetic center post as proposed by Chow and Pantell in the technical article xe2x80x9cThe Cyclotron Resonance Backward Wave Oscillator,xe2x80x9d documented in the proceedings of the IEEE, Vol. 48, pp. 1865-1867 (1980). In this technique, the center post carries the magnetic flux, while the magnetic envelope reduces the axial magnetic field on the cathode structure to virtually zero. However, a problem with this technique is that the magnetic center post is at essentially the same potential as that of the cathode; hence, practical implementations of this technique are prone to arcing between the center post and the anode due to large potential differences at their proximity. Moreover, this approach does not permit the flexibility of varying the beam canonical angular momentum spread to actively control the beam velocity spread for different applications of the gyrotron gun.
Accordingly, one object of the present invention, the double cusp gyro-gun, is to provide a gyrotron gun and a method of use thereof that have the flexibility of actively controlling the axial velocity vz spread so as to accommodate different applications of the gyrotron gun.
Another object of the present invention is to provide a gyrotron gun and a method of use thereof that actively control the gyrating electron""s beam transverse-to-axial velocity, xcex1=Vxe2x8axa5/vz, as well as the position of the electron""s beam guiding center, rg, so as to allow the gyrotron gun to be used for both small and large orbiting applications.
A still further object of the present invention is to provide a gyrotron gun and a method of use thereof that provide the flexibility for independently and simultaneously controlling the gyrating electron beam transverse-axial velocity ratio, xcex1=Vxe2x8axa5vz; the position of the electron""s beam guiding center, rg; as well as the spread of the axial velocity, vz.
The invention is directed to a gyrotron gun that is operated to independently and simultaneously control a gyrating electron beam transverse-to-axial velocity ratio, xcex1=vxe2x8axa5vz; the position of the electron""s beam guiding center, rg; as well as the spread of the axial velocity vz, thereby, allowing the gyrotron gun to be used for large-orbit, small-orbit and even linear-beam modes of operation.
The gyrotron gun generates and forms a beam of electrons manifesting electron gyrating around a guiding center and having rotational energy. This is so that efficient phase bunching will result from a resonant interaction between the electron gyration and transverse component of the electromagnetic wave in the ensuing beam-wave interaction circuit. The gyrotron gun comprises first, second and third field coils and first and second means for establishing an abrupt change in a magnetic field. The field coils and the devices for establishing an abrupt change are arranged to form three regions. The field coils are operated so that each supply a predetermined strength of an axial magnetic field to allow for the control of the gyrating electron beam transverse-to-axial velocity ratio, xcex1=Vxe2x8axa5/vz; the position of the electrons beam guiding center rg; and the spread of the axial velocity, vz, so that the gyrotron gun can be used for small and large-orbit and even linear-beam modes of operation.